The current practice of conventional infrared thermography uses infrared images taken at one instant in time in order to detect the presence of internal defects. An infrared camera is used to measure the temperature of the surface of an object and this surface temperature may vary over areas that contain an internal defect. The internal defect interrupts the flow of heat within the object, which can then be measured at the surface. This is a well-known and established measurement method.
Conventional infrared thermography, however, suffers from several limitations. First, an infrared camera does not measure temperature directly, but instead measures radiant flux emitted from an object. The radiant flux measured by the infrared camera is a function of both the temperature of an object (the quantity desired to be measured) and the emissivity of the object. It is impossible or very difficult to separate temperature variations from emissivity variations. Invalid measurements may result from this phenomenon as an infrared camera will measure two objects at the same temperature but each with different emissivity, as different temperatures.
Another limitation of conventional infrared thermography is that if measurements are not taken at the right instant in time, then a defect cannot be measured. The object needs to be measured at the proper moment in time while it is changing temperature in order to detect a defect. It is very difficult to know the optimal measurement time in advance, as this depends on a number of factors that are typically not known. As a result, many images taken with conventional infrared thermography do not show defects, even though defects are present in the object.
Conventional infrared thermography is typically performed using a hand-held camera. An operator takes images of localized sections of an object at an instant in time in order to assess the object. Conventional infrared thermography is also used on mobile platforms, such as a truck or van where an infrared camera is fixed to the vehicle and images are taken of an object as the vehicle is driven over the object (such as a bridge deck).
Active thermography has been used to better measure internal defects where a heat source is used to introduce a controlled amount of heat into an object and an infrared camera is used to monitor how the heat flows through the object. The use of the active heat source may produce increased sensitivity and greater accuracy in detection of internal defects. However, active thermography is typically used on small-scale objects and used to measure defects in objects with thin layers or near-surface defects.
Further, lock-in thermography has been used where collection of infrared images are synchronized or correlated with some external sensor data. This method seeks to correlate infrared signals with the external sensor data. This method has typically been used in laboratory systems for the measurement of small-scale objects. This method has been used to measure stress distributions in small localized areas and to find fatigue cracks in metals.